This invention relates to a method for fabricating a metallic duct assembly by welding, and in particular, to a method in which a tubular member or duct that is used to fabricate the duct assembly is formed and heat treated so as to resist heat-induced distortion when subsequently welded.
Lightweight, very high strength materials are finding increasing use in the fabrication of structural components. The switch to such materials has arisen because of the need to reduce the weight of such structural components without sacrificing strength. More specifically, ventilation ducting for certain applications is now being made of thin-walled, titanium alloy tubing that has been press formed from relatively thin-gauge sheet material. Sections of the tubing are welded together, or to a fitting or connector, to form a duct assembly.
In practice it has been found that a portion of the tubing in the heat affected zone of the weld becomes distorted during the welding process. This distortion adversely affects the fatigue life of the duct assembly. Accordingly, it would be desirable to have a method of fabricating metallic tubing that renders the tubing resistant to distortion when it is subsequently welded during fabrication of a duct assembly.
A technique known as "thermal sizing" has been used to shape and precisely dimension hollow, thin-walled articles, such as nuclear fuel channels. In the process of thermal sizing, the "sizing" force or pressure results from the differential thermal expansion between two dissimilar metals or alloys. The hollow, thin-walled article is formed of a metal or alloy having a known coefficient of thermal expansion. Before the sizing treatment, the thin-walled article is mounted on or surrounded by a mandrel that is formed of a material having a coefficient of thermal expansion that is significantly greater than that of the thin-walled article. When the thus assembled article and mandrel are heated to an elevated temperature, the mandrel expands at a faster rate than the thin-walled article, thereby exerting radially directed pressure on the article. The article and mandrel materials are selected such that within a preselected temperature range, the mandrel will expand to an external, or internal, cross-sectional dimension that corresponds to the desired internal, or external, cross-sectional dimension of the sized article.